FIG. 8 shows an example of an electrically assisted power steering apparatus. In this specification, a front-rear direction is based on the traveling direction of a vehicle. Therefore, in FIGS. 1, 5, 8, and 9, the left sides become the front and the right sides become the rear. As shown in FIG. 8, an electrically assisted power steering apparatus includes a steering wheel 1, a steering shaft 2 having a rear end portion to which the steering wheel 1 is fixed, a cylindrical steering column 3 supporting the steering column 3 therein such that the steering column 3 is rotatable, and a power assist apparatus 4. The motion of the steering wheel 1 is transmitted to an input shaft 8 of a steering gear unit 7 through the steering shaft 2, the power assist apparatus 4, a universal joint 5a, an intermediate shaft 6, and another universal joint 5b. If the input shaft 8 rotates, a pair of tie rods 9 disposed on both sides of the steering gear unit 7 is pushed and pulled, whereby a rudder angle according to the amount of operation of the steering wheel 1 is applied to left and right steered wheels.
As shown in FIGS. 9 and 10, the power assist apparatus 4 includes an electric motor 10 which is an auxiliary power source, a hollow metal housing 11, an output shaft 12 which is an assisted shaft, a torsion bar 13, a torque detector 14, and a worm speed reducer 15. The output shaft 12, the torsion bar 13, the torque detector 14, and the worm speed reducer 15 are disposed inside the housing 11. The housing 11 can be divided into two parts in the front-rear direction. In other words, the housing 11 includes a hollow first housing element 16 forming the intermediate portion and the rear end portion of the housing 11, and an annular second housing element 17 forming the front end portion of the housing 11. The front end portion of the first housing element 16 and the rear end portion of the second housing element 17 are fitted together with sufficient coaxiality being ensured, and are joined by a plurality of bolts 18 (two bolts in the example shown in FIGS. 9 and 10). The electric motor 10 is fixed to a side of the first housing element 16 by a plurality of bolts 19a. The front end portion of the steering column 3 is fixed to the rear end surface of the first housing element 16 by a plurality of bolts 19b. 
The output shaft 12 is disposed in the front-rear direction and is supported by first and second ball bearings 20, 21 such that the output shaft 12 is rotatable inside the housing 11. The outer ring 22 of the first ball bearing 20 is fitted into and fixed to a front end side portion of the intermediate portion of the first housing element 16, and the outer ring 23 of the second ball bearing 21 is fitted into and fixed to the second housing element 17. Further, the front end portion of the steering shaft 2 is connected to the output shaft 12 through the torsion bar 13. A part of the front end portion of the output shaft 12 protruding to the outside of the housing 11 is joined with the universal joint 5a (see FIG. 8).
The torque detector 14 includes a torque detecting sleeve 24, and an annular torque sensor 25 disposed around the torque detecting sleeve 24. The torque detecting sleeve 24 is fitted onto and fixed to the front end portion of the steering shaft 2 and then is disposed around the rear end portion of the output shaft 12. The torque sensor 25 includes a cylindrical sensor case 26 having inward flange portions at both ends in an axial direction, and a sensor coil held inside the sensor case 26. The torque sensor 25 is interposed between a step portion 27 provided at the inner circumferential surface of the first housing element 16 and a disc spring 28 superimposed on a side of the outer ring 22 of the first ball bearing 20, thereby being positioned in the axial direction, and is fitted into and fixed to the first housing element 16.
The worm speed reducer 15 includes a worm wheel 29, a worm shaft 30, and a worm 30 which is provided at the intermediate portion of the worm shaft 30 and is meshed with the worm wheel 29. The worm wheel 29 is fitted onto and fixed to a portion of the intermediate portion of the output shaft 12 between the first and second ball bearings 20, 21 by serration engagement or the like. Both end portions of the worm shaft 30 are supported by third and fourth ball bearings 32, 33 such that the worm shaft 30 is rotatable with respect to both end portions of the lower portion of the first housing element 16. The base portion of the worm shaft 30 (the upper end portion in FIG. 10) is joined with a drive shaft 34 of the electric motor 10 such that torque transmission is possible.
In the electrically assisted power steering apparatus having the above-mentioned configuration, if a torque is applied from the steering wheel 1 to the steering shaft 2, the torque detector 14 detects the direction and magnitude of the torque. In other words, if a torque is applied from the steering wheel 1 to the steering shaft 2, the torsion bar 13 connecting the steering shaft 2 and the output shaft 12 is elastically deformed in a twist direction. According to this elastic deformation, the steering shaft 2 and the output shaft 12 rotate relatively. The amount of relative rotation and the direction and magnitude of the torque have a correlation. Therefore, the torque detector 14 detects the direction and magnitude of the torque on the basis of the amount of relative rotation. On the basis of the torque detection result, the electric motor 10 rotates the output shaft 12 through the worm speed reducer 15 with a torque larger than the torque input from the steering wheel 1 to the steering shaft 2. Therefore, it is possible to reduce the operation force of the steering wheel 1 necessary for applying a rudder angle to the left and right steered wheels.
With respect to a power assist apparatus of an electrically assisted power steering apparatus, in order to reduce the weight and cost of a housing of the power assist apparatus, it has been considered to make the housing of a synthetic resin. For example, there has been proposed a structure in which with respect to a pair of housing elements forming a housing, one of the housing element is made of a metal and the other housing element is made of a synthetic resin (see, e.g., Patent Document 1). However, in this structure, in a state in which the end portions of the housing elements are fitted together to assemble the housing, the sufficient coaxiality of the housing elements may not be obtained. That is, shape accuracy and size accuracy of synthetic resin housing element are inferior to shape accuracy and size accuracy of a metal housing element. Accordingly, when sufficient shape accuracy and size accuracy of the synthetic resin housing element are not obtained, in a state in which the end portions of the housing elements are fitted together, sufficient coaxiality of the housing elements cannot be ensured.
Also, there has been proposed a structure in which two housing elements are made of a synthetic resin, and the end portions of the housing elements are welded (see e.g., Patent Document 2). In this case, because both of the housing elements are made of synthetic resin, there is higher likelihood of not being able to sufficiently ensure the coaxiality of the housing elements, for the reasons mentioned above.
In either case, if the coaxiality of the housing elements cannot sufficiently be ensured, also the coaxiality of components to be disposed concentrically to each other (the bearings, the assisted shaft, the torsion bar, the torque detecting sleeve, the torque sensor, and the like) among components attached to the inside of one housing element and components attached to the inside of the other housing element cannot sufficiently be ensured. As a result, during the operation of the power assist apparatus, an excessive force may be applied to each corresponding component, whereby the durability of those components may be reduced or the accuracy of the torque detection of the torque detector may be lowered. Further, there is also a possibility of not being able to assemble the power assist apparatus.